I think he'll be fine. Pierce is actually better in 3s and while Monta is better in steals and assists, Pierce is pretty solid there as well. This gives you a nice boost in FT, TOS, Rebs as well. Small edge to Pierce in blocks too. Points should be a wash. So you just lose about a half a steal a game and a couple assists for some nice boosts across the board.

I wouldn't worry to much about his age. Doc is one of the better coaches in the league at limiting his older players' minutes. Just look at KG and his 5 minute on 5 minute off rotation.